how will you know if the guy cheating.

If he said it was his boss, why not give him the benefit of the doubt?

It might well be the truth. My boyfriend sometimes walks away whilst on the phone, but it is because he does not wish to talk in a setting he deems inappropriate. This really just depends on the context. Were they having dinner in a restaurant? If so maybe it was too noisy or he felt rude talking over the dinner table. Or maybe he was just organising a surprise for her and he did not want her to know!! Also it is fairly hard to tell if somebody is cheating from one incident. She needs to look for other signs. Staying out late all the time, overly or under responsive, showers of presents to make up for the guilt, always taking calls and messages elsewhere. My advice was to analyse his general behaviour and see if it has dramatically changed, but not to be overly observant or else you can fool yourself into false conclusions. Hope this helps!!